The invention relates to a manufacturing process for die-cast light-metal parts, particularly for die-cast light-metal wheels of passenger cars, using an almost eutectic refined AlSi alloy, which, in addition to Al, contains parts by weight of approximately 9.5% to 12.5% silicon and other low alloying constituents, such as magnesium, iron, manganese, titanium, copper, zinc, as well as no more than 0.05 each and a total of no more than 0.15 other impurities. The parts, after die-casting and solidification, are removed from the casting die and cooled. The alloy, which is used in the manufacturing of parts of this type and particularly of passenger car wheels, contains proportions of hardenable alloying constituents, which are kept as low as possible, because these hardenable alloying constituents reduce the crashworthiness or ductility of the parts. However, in order to be able to further reduce wall thickness and the weight of the parts, the requirement exists to simultaneously improve their strength as well as their ductility, or to at least achieve an improvement in one of these two characteristics.
The invention achieves a surprisingly high increase of the strength without any reduction, or even with a simultaneous slight increase in ductility, in a surprisingly simple manner by having the alloy contain at least 0.05 to no more than 0.15% parts by weight of magnesium. The parts' interior areas and areas with concentrations of mass, such as the hub and the disk of wheels, are quenched in water immediately after their removal from the die and before a temperature measured at their surfaces is below at least 380.degree.. This type of a narrow and low defining of the magnesium content and other hardenable constituents, together with a quench hardening of such an alloy, has up to now not been considered useful and has therefore not been used.
Although a less targeted cooling of similarly alloyed wheels in water, immediately after the removal from the die is known for the purpose of a subsequent easier handling, there is no known targeted improvement of the strength for these parts that would correspond to the results of the invention.
Particularly in the case of die-cast light-metal wheels for passenger cars, an increase of up to approximately four times the rotary bending fatigue limit, which is important, can be achieved by the process according to the invention in the hub area and the wheel disk area. A simultaneous up to 25% increase of the impact ductility at the rim flange can be obtained if the thin-walled impact-resistant rim flange area has its quenching take place after the temperature reaches 400.degree. C.
In connection with an alloy of the GK-AlSill type, according to DIN (German Standard) 1725, Part 2 and 5/2.86 which contains parts by weight in percent of no more than 10.5 Si, 0.1 Mg, 0.2 Fe, 0.05 Mn, 0.1 Ti and 0.05 Zn and a strontium refinement with approximately 0.025% Sr, and a grain refining treatment with approximately 0.1% of an aluminum titanium intermediate alloy, AlTi.sub.5 B.sub.1, is provided with narrow tolerances. The temperature at the time of the quenching which differ at the hub and rim flange areas as a result of the different cooling speed, should be approximately 400.degree. C. to 500.degree. C. in the hub area and approximately 325.degree. C. to 375.degree. C. in the rim flange area. This results in an increase of the strength and of the ductility which is advantageous in view of the respective purpose of the casting for wheels for vehicles. An appropriate application of these relationships to other casting parts having thin walls is within the scope of this invention.
When the die-casting takes place merely at a molten mass of no more than 720.degree. in the melting furnace and of no more than 680.degree. C. in the holding furnace, the strength and the ductility will increase, to approximately the same extent, i.e., by approximately 50%. If the quenching of interior areas and areas with concentrations of mass takes place before surface temperatures thereat reach 380.degree. C. and the temperature is approximately 400.degree. C. to 500.degree. C. in the hub area and 325.degree. C. to 375.degree. C. in the rim area and with a molten mass of no more than 720.degree. C. in the melting furnace and of no more than 680.degree. C. in the holding furnace, an additional increase of at least the ductility from approximately 25% to approximately 50% is obtained.
If the parts or wheels are subject to artificial aging or tension removal at a temperature of approximately 190.degree. C. for a duration of approximately 20 minutes, perhaps during a surface coating or painting operation, one avoids, after the quench hardening, a remaining tension, which causes the formation of cracks. Practical tests have led to the also surprising result that a sufficient artificial aging is achieved merely by the heat treatment, which is customary in the case of unquenched parts or wheels during the drying phase of a painting process, so that no additional heating energy is required for a separate heat treatment.